Multi-functional solder and articles made therewith, such as microelectronic components

ABSTRACT

Aspects of the invention provide solder compositions which include two different fluxing agents. One of the fluxing agents promotes melting of a metal of the solder at a first activation temperature and the other fluxing agent promotes melting of the metal at a second activation temperature that is higher than the first activation temperature. This dual-flux solder may be used in manufacturing microelectronic components and microelectronic component assemblies. In one specific application, the solder may be used to manufacture a flip chip or other microelectronic component which includes self-fluxing solder balls. This can obviate the need to apply another flux composition to the solder balls prior to a subsequent component attach reflow operation.

TECHNICAL FIELD

The present invention provides certain improvements in processingmicroelectronic workpieces. The invention has particular utility inconnection with microelectronic component processing, e.g., inmanufacturing microelectronic devices including solder connections.

BACKGROUND

Different components of a microelectronic component assembly can beattached and electrically coupled to each other in a variety of ways.For example, flip chips typically include a series of bond pads, each ofwhich carries a separate solder ball or “bump.” This “bumped” chip maythen be positioned on a substrate with the solder balls contacting anarray of electrical contacts carried on a surface of the substrate. Byheating the solder, the flip chip can be mechanically joined andelectrically coupled to the substrate.

FIG. 1 schematically outlines a conventional process for “bumping” aflip chip and attaching the flip chip to a substrate. This process 10starts with a chip bearing a plurality of bond pads in step 12. Solderis deposited on the bond pads in step 14. This may be accomplished, forexample, by depositing a solder paste through a solder stencil or byattaching pre-formed solder spheres on the bond pads. Solder pastescommonly include a soldering flux which reacts with metal oxides in thesolder paste, which can promote melting of the solder metal and wettingof the bond pads with the solder when the solder is heated above areflow temperature. If solder spheres are used, the solder spheres aretypically contacted with a tacky flux composition which helps hold thesolder spheres in place as they are heated to a temperature to whichthey will flow. This heating process, commonly referred to as “reflow,”will metallurgically attach the solder to the bond pads, creating solderballs or bumps on the chip (step 16).

The solder bumps will tend to oxidize. Accordingly, a flux is usuallynecessary to clean the metal oxides from the solder to attach the chipto the substrate. In step 20, the solder balls are dipped in a flux.This is commonly accomplished using a rotating drum with a supply of anorganic flux and a doctor knife to control the thickness of the flux.Thereafter, the chip may be placed on the substrate in step 22. Thesolder balls on the chip may be reflowed to attach the component to thesubstrate (step 24), with the flux deposited on the solder balls in step20 promoting flow of the solder.

Older fluxes commonly left a residue that was unsightly and/orinterfered with further processing of the microelectronic assembly ordegraded its performance. Accordingly, such flux residues may be cleanedin step 26. Increasingly, so-called “no-clean” fluxes are beingemployed. Such fluxes typically leave virtually no reside at all orleave a residue which is unlikely to interfere with further processingor use of the microelectronic assembly.

When the flip chip is coupled to the substrate, a gap is commonly leftbetween the flip chip and the opposed surface of the substrate. Tofurther enhance the mechanical bond between the chip and the substrate,this gap may be filled in step 28 with an underfill material, typicallyan organic resin. This underfill material may also protect the solderjoints from chemical attack by moisture or other agents. At the end ofthe process 10, optionally including the cleaning and underfill steps 26and 28, a microelectronic component assembly is produced (step 40).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of a conventional flip chipmanufacturing process in accordance with the prior art.

FIG. 2 is a schematic flow diagram of a method in accordance with oneembodiment of the present invention.

FIG. 3 schematically illustrates a microelectronic component in onestage of the process shown in FIG. 2.

FIG. 4 schematically illustrates the microelectronic component of FIG. 3in a later stage of the process shown in FIG. 2.

FIG. 5 is a schematic illustration of the microelectronic component ofFIG. 4 incorporated in a microelectronic component assembly in a laterstage of the process shown in FIG. 2.

FIG. 6 schematically illustrates the microelectronic component assemblyof FIG. 5 in a later stage of the process shown in FIG. 2.

FIG. 7 graphically illustrates heating profiles that may be used inheating and in reheating a solder composition in one embodiment of theinvention.

DETAILED DESCRIPTION

A. Overview

Various embodiments of the present invention provide soldercompositions, microelectronic components, microelectronic componentassemblies, and methods employing such microelectronic components.Unless the specific context clearly requires otherwise, throughout thedescription and claims the terms “microelectronic component” and“microelectronic component assembly” may encompass a variety of articlesof manufacture, including, e.g., semiconductor wafers having activecomponents, individual integrated circuit dies, packaged dies, andsubassemblies consisting of two or more microelectronic components,e.g., a stacked die package. The following description provides specificdetails of certain embodiments of the invention illustrated in thedrawings to provide a thorough understanding of those embodiments. Itshould be recognized, however, that the present invention can bereflected in additional embodiments and the invention may be practicedwithout some of the details in the following description.

In one embodiment, the present invention provides a solder compositionadapted to couple a microelectronic component to a substrate. Thissolder composition may include an electrically conductive metal and aflux composition. The flux composition includes a first fluxing agentand a second fluxing agent. The first fluxing agent has flux activity,e.g., it reacts with an oxide of the metal, at a first activationtemperature and is present in an amount to promote a first melting ofthe metal at the first activation temperature. The second fluxing agenthas a melting temperature higher than the first activation temperatureand has flux activity, e.g., reacts with an oxide of the metal, at asecond activation temperature that is higher than the first activationtemperature. The second fluxing agent is present in an amount to promotea second melting of the metal at the second activation temperature whichfollows the first melting and a subsequent resolidification of themetal.

In another embodiment, the invention provides a microelectroniccomponent including such solder. In particular, the microelectroniccomponent includes a first surface bearing a plurality of electricalterminals. The component also includes a plurality of solder deposits,each solder deposit being positioned on a different one of theelectrical terminals. The solder may have the same composition as thatmentioned in the preceding paragraph.

Another embodiment of the invention provides a solder compositionadapted to couple a microelectronic component to a substrate. The soldercomposition includes a metal alloy and a flux composition. The fluxcomposition comprises a binder, a thixotropic rheology modifier, a firstfluxing means for reacting with an oxide of the alloy to promote meltingof the alloy at a first reflow temperature, and a second fluxing meansfor reacting with an oxide of the alloy at a higher second reflowtemperature, which second melting occurs after the first melting of thealloy and a subsequent resolidification of the alloy.

Still another embodiment of the invention provides a microelectroniccomponent which comprises a body having a first surface bearing aplurality of electrical terminals formed of a first electricallyconductive material and a plurality of solder balls. Each solder ball isattached to a different one of the electrical terminals (e.g., by aball-attach reflow) and each solder ball comprises an electricallyconductive metal and a solid fluxing agent within the solder ball. Theelectrically conductive metal has a metal flow temperature and is adifferent material from the first electrically conductive material ofthe terminals. The solid fluxing agent has flux activity at a reflowtemperature, which may be higher than the metal flow temperature. Thefluxing agent is present in an amount efficacious to promote melting ofthe metal at the reflow temperature.

An alternative embodiment of the invention provides a microelectroniccomponent assembly that includes a microelectronic component and amicroelectronic substrate. The microelectronic component has a firstconfronting surface bearing a plurality of component electricalterminals formed of a first electrically conductive material. Themicroelectronic substrate has a second confronting surface bearing aplurality of substrate electrical terminals formed of a secondelectrically conductive material. The second confronting surface isopposed to the first confronting surface. A plurality of solder ballsare disposed between the first and second confronting surfaces, witheach solder ball being attached to a different one of the componentelectrical terminals and abutting, but not bonded to, one of thesubstrate electrical terminals. Each solder ball may comprise anelectrically conductive metal that has a metal flow temperature and asolid fluxing agent within the solder ball. The electrically conductivemetal of the solder balls may be different from the first electricallyconductive material and from the second electrically conductivematerial. The solid fluxing agent has flux activity at a componentattach reflow temperature, which may be higher than the metal flowtemperature. The fluxing agent may be present in an amount efficaciousto promote melting of the metal at the component attach reflowtemperature.

Still another embodiment of the invention provides a method ofprocessing a microelectronic component that has a first surface bearinga plurality of component electrical terminals. In accordance with thismethod, a plurality of solder deposits are deposited, with each solderdeposit being deposited on a different one of the component electricalterminals and comprising an electrically conductive metal, a firstfluxing agent, and a second fluxing agent. The solder balls are heatedto a ball attach reflow temperature and the first fluxing agent reactswith an oxide of the electrically conductive metal to promote flow ofthe solder deposits at the ball attach reflow temperature. These solderdeposits are allowed to cool to a temperature below the ball attachreflow temperature, with each cooled solder deposit defining a solderball attached to a component electrical terminal. Each solder ballincludes an amount of the second fluxing agent efficacious to promoteflow of the solder at a temperature higher than the ball attach reflowtemperature.

Yet another embodiment of the invention provides a method ofelectrically coupling a microelectronic component having componentelectrical terminals to a microelectronic substrate having substrateelectrical terminals. This method includes depositing solder on at leastsome of the component electrical terminals, with the solder comprisingan electrically conductive metal, a first fluxing agent, and a secondfluxing agent. The solder deposits are heated to a ball attach reflowtemperature and the first fluxing agent promotes flow of the solderdeposits at the ball attach reflow temperature. The solder deposits areallowed to cool to a temperature below the ball attach reflowtemperature, with each cooled solder deposit defining a solid solderball attached to a component electrical terminal. Each solder ballincludes an amount of the second fluxing agent. The microelectroniccomponent and the microelectronic substrate are juxtaposed, with each ofthe solder balls contacting at least one of the substrate electricalterminals. The solder balls are heated to a second reflow temperaturethat is higher than the ball attach reflow temperature. The second fluxpromotes flow of the solder at the second reflow temperature toelectrically couple the microelectronic component to the microelectronicsubstrate.

B. Solder Compositions

Some embodiments of the invention provide solder compositions that maybe used to couple a microelectronic component to a substrate. The soldercompositions may generally include an electrically conductive metal anda flux composition. A wide variety of electrically conductive metals maybe suitable. In one embodiment, the metal is selected to form a stablemetallurgical bond with the material of an electrical contact to whichthe solder will be attached. The metal of the solder may also beselected to be mechanically and chemically compatible with the otherfeatures of the microelectronic component.

The electrically conductive metal may comprise a single metallicelement. More commonly, the electrically conductive metal will comprisean alloy. In one embodiment, the solder is an alloy comprising zinc andlead. The alloy optionally includes other elements, as well, such assilver. In one embodiment, the metal comprises a eutectic consistingessentially of about 63% tin and about 37% lead. In another embodiment,the metal alloy consists essentially of about 62% tin, about 36% lead,and about 2% silver. Lead-free solder compositions are gaining increasedacceptance in the microelectronics industry. Hence, in alternativeembodiments, the metal may comprise one or more of Sn—Ag—Cu alloys,Sn—Zn—Bi alloys, and Sn—Ag—Bi alloys. Any other suitable electricallyconductive metal may instead be used; a wide variety of metals and metalalloys are commercially available for use as microelectronic componentsolders.

If the electrically conductive metal comprises a eutectic alloy, themetal may exhibit a melting point at a specific temperature. Withnon-eutectic compositions, the metal will commonly melt over a range oftemperatures; in such metals, the solidus may be considered the meltingtemperature even though solid and liquid metal may coexist.

The electrically conductive metal may be present in the soldercomposition in a variety of forms. In one embodiment, the electricallyconductive metal in the solder composition comprises a finely dividedpowder having a particle size of between about 10 and about 100 microns,with a range of about 20–75 microns sufficing for most applications. Themetal may comprise a homogenous powder in which each particle of thepowder has a substantially homogenous composition. In anotherembodiment, the metal particles are non-homogenous and may include acoating of a lower melting point metal surrounding a higher meltingpoint core. Solder particles that more closely approximate a sphericalshape may improve processing parameters during solder reflow.

As noted above, the solder composition also includes a flux composition.In one embodiment, the flux composition comprises a first fluxing agent,a second fluxing agent, and a flux vehicle including a binder, asolvent, and a rheology modifier. In some applications, the flux vehiclemay omit one or more of these elements and, in select circumstances, theflux vehicle may be entirely omitted.

Flux vehicles are well known in the art and any of a wide variety ofcommercially available binders, solvents, and rheology modifiers may beemployed in the flux vehicle of the present solder. The nature andrelative percentages of these materials in the solder composition willvary depending on the particular application. For example, the slumpproperties and tackiness of a solder composition used in non-fine pitchstencil printing applications may be materially different from therequirements for a solder composition to be deposited by screen printingor using a syringe.

As flux vehicles are well known, there is no need to explain them ingreat detail here. Briefly, though, the binder may comprise a rosin,which is (or is derived from) a naturally occurring material found inconiferous trees, or a man-made resin. This rosin or resin may beselected to flow at a temperature that is less than the temperature atwhich the solder composition will begin to flow. The solvent may act asa carrier system that will be volatilized after application. Theselection of the solvent will depend on the desired work life, tacktime, and slump of the solder composition and will also affect theheating profile requirements. The rheology modifier will affect the flowproperties of the solder composition and may also limit separation ofthe solder composition during long-term storage. In one embodiment, therheology modifier is thixotropic; a number of thixotropic rheologymodifiers are known in the art.

Flux compositions in accordance with embodiments of the invention employa first fluxing agent which promotes melting of the metal in the soldercomposition at a first activation temperature and a second fluxing agentwhich promotes melting of the metal at a higher second activationtemperature. Both of these fluxing agents are adapted to promote meltingand flow of the metal in the solder by reacting with contaminants in thesolder composition that may inhibit such melting. For example, theparticles of electrically conductive metal may have a relatively highsurface area, increasing the likelihood that the metal will oxidize.These metal oxides and a number of other potential contaminants arerefractory materials that will make it more difficult to melt the metalparticles. By reacting with the metal oxides and/or other contaminantsthat can interfere with efficient melting of the metal particles, thefluxing agents can promote the flow of the solder composition at a lowertemperature. In one embodiment, the first and second fluxing agents bothalso limit reaction of the atmosphere with the molten electricallyconductive metal, e.g., during a reflow operation.

One difference between the first and second fluxing agents is thetemperature at which the fluxing agent significantly promotes melting ofthe electrically conductive metal and flow of the solder composition.The first fluxing agent may react with oxides and other contaminants ata first temperature that is at or below the melting point of theelectrically conductive metal. This will help remove the oxides andother contaminants that can interfere with efficient melting of themetal so it can begin melting upon reaching its melting point. Inanother embodiment, the first fluxing agent exhibits fluxing propertiesat temperatures above the melting point of the metal. In one embodiment,the first activation temperature is no higher than about 240° C., e.g.,about 150° C.–220° C. Such a fluxing agent may be appropriate forlead-tin eutectic alloys, which have a melting point of about 183° C. Inlead-free solder compositions, where the melting point of the metalalloy tends to be higher, the first fluxing agent may exhibit fluxingactivity only at higher temperatures.

The first fluxing agent may comprise any of a number of common fluxingagents with fluxing activity at the desired temperature. In oneembodiment, the first fluxing agent is selected from a group consistingof organic acids, amines, alcohols, and epoxy resins with across-linking agent with fluxing properties. Suitable organic acidsinclude, but are not limited to, carboxylic acids (—COOH). Suitableamine fluxing agents include, but are not limited to, aliphatic amineshaving 1–10 carbon atoms, e.g., trimethylamine, triethylamine,n-propylamine, n-butylamine, isobutylamine, sec-butylamine,t-butylamine, n-amylamine, sec-amylamine, 2-ethylbutylamine,n-heptylamine, 2-ethylhexylamine, n-octylamine, and t-octylamine.Various epoxy resins employing a cross-linking agent with fluxingproperties and useful as the first fluxing agent are disclosed in U.S.Pat. No. 6,367,150, the entirety of which is incorporated herein byreference. In one embodiment, the first fluxing agent comprises amixture of two or more fluxing agents, each of which reacts with anoxide of the electrically conductive metal at a temperature at or belowthe desired first activation temperature.

The first fluxing agent exhibits flux activity, e.g., it reacts withoxides of the metal in the solder composition, at a lower temperaturethan the second fluxing agent exhibits flux activity. Hence, the firstactivation temperature is lower than the second activation temperature.In one embodiment, the second activation temperature is at least about25° C. higher than the first activation temperature. A second activationtemperature of about 25–70° C. higher than the first activationtemperature is expected to suffice for most applications. In oneembodiment, the second activation temperature is higher than a meltingtemperature of the electrically conductive metal. It is anticipated thata temperature difference between the first and second activationtemperatures of less than 25° C. may require unduly rigorous processcontrols in some applications. In one particular embodiment, the secondactivation temperature is at least about 200° C.; a second activationtemperature of about 220–270° C. is expected to work well with mostSn—Pb-based solder compositions. As noted above, lead-free soldercompositions may require higher processing temperatures.

Any of a variety of commercially available fluxing agents may beemployed as the second fluxing agent. In one particular embodiment, thesecond fluxing agent has a melting temperature that is higher than thefirst activation temperature. In one embodiment, the second fluxingagent is selected from a group consisting of organic acids, amines,alcohols, and epoxy resins with a cross-linking agent with fluxingproperties. In one application of this embodiment, the second fluxingagent includes at least one of an organic acid with two or morecarboxylic groups, and an epoxy resin with a cross-linking agent withfluxing properties. In one more specific example, the second fluxingagent comprises an agent selected from a group consisting of aromaticdicarboxylic acids having 6–30 atoms and aliphatic dicarboxylic acidshaving 6–30 carbon atoms. In some embodiments, the second fluxing agentcomprises a “no-clean” fluxing agent that either leaves no substantialresidue or leaves a residue that is compatible with a resinous underfillmaterial. U.S. Pat. No. 6,059,894, the entirety of which is incorporatedherein by reference, suggests a variety of monocarboxylic,hydroxycarboxylic, and dicarboxylic acids which, used alone or mixedwith one another, may provide a suitable second fluxing agent; some ofthese acids may also be useful as a first fluxing agent.

Hence, embodiments of the present invention provide solder compositionsthat include a metal, a lower-temperature fluxing agent, and ahigher-temperature fluxing agent. By appropriately selecting theactivation temperatures of the two fluxing agents, one can heat thesolder to a temperature at or above both the first activationtemperature and the melting temperature of the metal, yet not reach orexceed the second activation temperature. This can facilitate a firstsoldering operation, such as a solder reflow.

The second fluxing agent should be stable enough at the temperature ofthe first soldering operation that a residual quantity of the secondfluxing agent will remain in the hardened solder at the end of the firstsolder operation. Not all of the second fluxing agent need survive thefirst soldering operation. In one embodiment, though, the second fluxingagent should be present in the solder after the first solderingoperation in an amount efficacious to promote a second melting of themetal. If the second fluxing agent is solid in the range of temperaturesencountered in the first soldering operation, for example, this secondfluxing agent may be activated by heating it to a temperature at orabove its melting temperature.

C. Selected Applications for Solder Composition

Various embodiments of the solder composition described above can beused in a number of different operations. FIG. 2 schematicallyillustrates a method of assembling a microelectronic component assemblyin accordance with one embodiment of the invention. In this method 100,a microelectronic component is provided at step 112. The microelectroniccomponent, as noted above, can take any of a variety of forms. It may,for example, comprise a semiconductor wafer, a bare or packagedintegrated circuit die or chip, or a multi-component microelectronicsubassembly. One such microelectronic component 210 is shown in FIG. 3.This microelectronic component 210 has a plurality of electricalterminals 214 arrayed on a terminal surface 212. In one embodiment, asoldermask is provided on the entire terminal surface 212 except for theareas of the electrical terminals 214. Such a soldermask may limitwetting of the terminal surface 212 by the solder in a subsequent reflowoperation. A variety of soldermasks are known in the art andcommercially available. By way of example, one possible soldermask isdisclosed in U.S. Pat. No. 6,388,199, the entirety of which isincorporated herein by reference.

In step 214 of FIG. 2, a solder comprising a two-stage flux compositionsuch as those discussed above may be deposited on the terminals 214. Asshown in FIG. 3, this solder may be delivered using a conventionalstencil to place a solder deposit 220 atop each of the terminals 214. Inother embodiments, the solder composition may be applied as a solidsolder ball which is held on the terminals 214 using a suitable tackymaterial, or may be delivered via silk screening, a syringe, or anyother known technique.

In step 116 of FIG. 2, the solder deposits 220 are reflowed at a firsttemperature. In one embodiment, this first temperature is at least asgreat as the melting temperature of the metal of the solder; in a morespecific embodiment, this first temperature exceeds the meltingtemperature of the metal. At this first temperature, the first fluxingagent is able to react with an oxide of the metal of the soldercomposition, promoting melting and, hence, flow of the metal. This firstfluxing agent may also be adapted to clean the surface of the terminals214, e.g., by reducing any oxides thereon or helping remove apassivation layer which may be applied on the terminals 214.

After the solder has been reflowed in step 116, it may be allowed tocool to a temperature below the melting temperature of the solder. If sodesired, the solder and the microelectronic component 210 may be allowedto cool to room temperature for storage. As illustrated in FIG. 4, thesolder deposits 220 of the subassembly 200 shown in FIG. 3 may formsolder balls 222, with one solder ball 222 associated with each terminal214. FIG. 4 illustrates these solder balls 222, which are alsoconventionally referred to as solder bumps, as idealized frustrums ofspheres. It should be recognized that other shapes are possible, though.If the flux composition, including the first fluxing agent, is not a“no-clean” composition, the resultant bumped microelectronic component201 may be washed to remove the flux residue.

The bumped microelectronic component 201 may have an appearance thatresembles that of a conventional bumped flip chip. In a conventionalbumped flip chip, however, relatively little or none of the fluxingagent will remain on or in the solder balls. In particular, theregenerally is not sufficient flux associated with the solder balls toeffectively promote reflow of the solder balls in the subsequent reflowstep (24 in FIG. 1). As a consequence, the solder balls will require anadditional external application of flux, such as by dipping the solderballs in a supply of flux, as discussed above in connection with step 20of FIG. 1.

In contrast, the solder balls 222 of the microelectronic component 201shown in FIG. 4 may be thought of as “self-fluxing” solder balls. Inparticular, the second fluxing agent is present in the solder balls 222in an amount efficacious to promote melting of the solder balls in asubsequent reflow operation. In one embodiment, at least a portion ofthe second fluxing agent is dispersed in the body of the solder ball222, though some of the second fluxing agent may migrate to the surfaceof the solder balls 222 during the ball-attach reflow 116. The presenceof the second fluxing agent in the solder balls 222 may obviate the needto apply additional solder, as is required with a conventional flip chipoperation. Eliminating the additional handling necessary to dip thesolder balls in a separate flux can increase throughput of themanufacturing operation and reduce product losses attributable to thehandling operation.

This bumped microelectronic component 201 may be positioned with respectto a substrate in step 118 of FIG. 2. As shown in FIG. 5, the substrate240 may include a confronting face 242 that carries an array ofsubstrate electrical terminals 244. At least some of these terminals maybe arranged in an array that mirrors the array of terminals 214 andattached solder balls 222 of the microelectronic component 201.

The substrate 240 may be rigid or flexible and have any desiredconfiguration. The substrate 240 may be formed of material commonly usedto manufacture microelectronic substrates, such as ceramic, silicon,glass, or combinations thereof. The substrate 240 can alternatively beformed of an organic material or other materials suitable for PCBs; inone embodiment, the substrate 240 comprises an FR-4 PCB. In anotherembodiment, the substrate 240 comprises a flexible interposer, such as aconventional polyamide tape. In other embodiments, the substrate 240 maycomprise another microelectronic component and the resultantmicroelectronic assembly 250 (FIG. 6) may be a stacked die package usedin a subsequent assembly operation.

As illustrated in FIG. 5, the solder balls 222 are positioned betweenthe terminal surface 212 of the microelectronic component 210 and theconfronting face 242 of the substrate 240. At this stage, the solderballs 222 will not be bonded to the substrate terminals 244. Each of thesolder balls 222 may simply abut the terminals 244 of the substrate 240.

With solder balls 222 of the bumped microelectronic component 201positioned in contact with or adjacent to the terminals 244 of thesubstrate, the solder may be heated to a second temperature to reflowthe solder in step 120 (FIG. 2). In one embodiment, this secondtemperature is higher than the first temperature in step 116 and mayexceed any temperature encountered in step 116. Hence, it is anticipatedthat this second temperature will exceed the melting temperature of themetal in the solder composition.

By way of example, FIG. 7 illustrates a pair of heating profiles thatmay be useful in one specific embodiment of the invention. In thisexample, the lower curve 300 is an exemplary heating profile for thefirst reflow operation (116 in FIG. 2) and the upper curve 320represents an exemplary heating profile for the second reflow operation(120 in FIG. 2). Looking first at the lower curve 300, the temperaturein the first reflow operation may be ramped up in the first heatingphase 302 and held relatively stable in a second soak phase 304. Thiscan help preheat the microelectronic component 210 and reduce anyproblems that may arise from different coefficients of thermal expansionof the materials in the microelectronic component 210. If a flux vehicleincluding a resin binder and a solvent are employed, this preheat 302,304 can also help evaporate the solvent and melt or decompose thebinder. After the preheat 302, 304, the temperature may be increasedagain in a reflow ramp 306. This reflow ramp 306 may continue to amaximum temperature 308 that meets or exceeds the first activationtemperature.

In the particular example of FIG. 7, in which the solder may comprise alead-tin eutectic alloy having a melting point of about 183° C., themaximum temperature 308 is about 212° C. This will allow a sufficientdwell time d, at or above 183° C., the melting point of the metal, toallow the solder deposits 220 (FIG. 3) to wet the terminals 214 of themicroelectronic component 210 and form the solder balls 222. After themaximum temperature 308 is reached, the temperature may be decreased ina cool-down phase 310. This will cause the solder balls 222 to solidify,generally as shown in FIG. 4.

The upper curve 320 in FIG. 7 provides an exemplary heating profile forthe second reflow operation 120 (FIG. 2). Much like the lower curve 300,the upper curve 320 can include a preheat operation involving a firstheating phase 322 and a second soak phase 324. After this preheatoperation 322, 324, the temperature may be ramped up again in a reflowramp 326 to a maximum temperature 328. This maximum temperature 328 isgreater than both the maximum temperature 308 of the first curve 300 andthe melting point (183° C.) of the metal in the solder. Thereafter, thesolder may be allowed to cool in a cool-down phase 330. In theillustrated embodiment, this provides a dwell time d₂ that is slighterlonger than the dwell time d₁ of the lower curve 300. In anotherembodiment, the second dwell time d₂ is equal to or less than the firstdwell time d₁.

The line 335 in FIG. 7 schematically illustrates a melting temperatureof the second fluxing agent in the solder composition. This temperature335 is greater than the maximum temperature 308 of the first curve 300.Consequently, the second fluxing agent may remain substantially solidthroughout the first heating process 300. The maximum temperature 328 inthe second heating process 320, however, exceeds this meltingtemperature 335. As a consequence, the second fluxing agent may melt,making it more reactive with the oxides or other contaminants in thesolder balls 222, enabling it to more readily promote melting of thelead-tin alloy and flowing of the solder balls 222.

As illustrated in FIG. 6, the resultant microelectronic componentassembly 250 has a plurality of solder bridges 224 which aremechanically bonded to and electrically interconnect the terminals 214of the microelectronic component 210 and the terminals 244 of thesubstrate 240. These bridges 224 may include a residue or reactionbyproduct of the second fluxing agent and may also include a remainingunconsumed amount of the second fluxing agent. In one embodiment,though, little or none of the second fluxing agent will remain in thebridges 224.

Turning back to FIG. 2, any remaining flux residue on the bridges 224 orother components of the microelectronic component assembly 250 may becleaned in step 122. In step 124, the gap 245 between themicroelectronic component 210 and the substrate 240 may be filled withan underfill material. A wide variety of resins and epoxies are wellknown in the art for such underfill operations.

Hence, at the end of the process 100, a finished microelectroniccomponent assembly 250 is provided (step 140). FIGS. 3–6 illustrate anembodiment of the method 100 of FIG. 2 in the context of a flipchip-type manufacturing operation. It should be recognized, however,that the self-fluxing solder balls 222 that result from the ball-attachreflow step 116 can be useful in a wide variety of applications. Thismay be helpful, for example, if the microelectronic component 210comprises a BGA package (e.g., the BGA package illustrated in U.S. Pat.No. 6,388,199, noted above).

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in a sense of “including,but not limited to.” Words using the singular or plural number alsoinclude the plural or singular number respectively. The above detaileddescriptions of embodiments of the invention are not intended to beexhaustive or to limit the invention to the precise form disclosedabove. While specific embodiments of, and examples for, the inventionare described above for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize. For example, while steps arepresented in a given order, alternative embodiments may perform steps ina different order. Aspects of the invention may also be useful in otherapplications, e.g., in soldering processes that physically join elementsbut do not require electrical connections. The various embodimentsdescribed herein can be combined to provide further embodiments.

In general, the terms used in the following claims should not beconstrued to limit the invention to the specific embodiments disclosedin the specification, unless the above detailed description explicitlydefines such terms. While certain aspects of the invention are presentedbelow in certain claim forms, the inventors contemplate the variousaspects of the invention in any number of claim forms. Accordingly, theinventors reserve the right to add additional claims after filing theapplication to pursue such additional claim forms for other aspects ofthe invention.

1. A solder composition adapted to couple a microelectronic component toa substrate, comprising: an electrically conductive metal; a firstfluxing agent reacting with an oxide of the metal at a first temperatureand present in an amount to promote a first melting of the metal at thefirst temperature; and a second fluxing agent having a meltingtemperature higher than the first temperature and reacting with an oxideof the metal at a second temperature that is higher than the firsttemperature, the second fluxing agent being present in an amount topromote a second melting of the metal at the second temperature afterthe first melting of the metal and a subsequent resolidification of themetal.
 2. The solder composition of claim 1 wherein the flux soldercomposition further comprises a flux vehicle including a binder, asolvent, and a rheology modifier.
 3. The solder composition of claim 2wherein the binder comprises a rosin or a resin adapted to flow at atemperature less than the first temperature.
 4. The soider compositionof claim 1 wherein the metal comprises an alloy of lead and tin.
 5. Thesolder composition of claim 1 wherein the metal is substantiallylead-free.
 6. The solder composition of claim 1 wherein the secondtemperature is at least about 25° C. higher than the first temperature.7. The solder composition of claim 1 wherein the second temperature isabout 25–70° C. higher than the first temperature.
 8. The soldercomposition of claim 1 wherein the first temperature is no higher thanabout 240° C.
 9. The solder composition of claim 1 wherein the firsttemperature is about 150–220° C.
 10. The solder composition of claim 1wherein the second temperature is at least about 200° C.
 11. The soldercomposition of claim 1 wherein the second temperature is about 220–270°C.
 12. The solder composition of claim 1 wherein the first fluxing agentcomprises an agent selected from a group consisting of organic acids,amines, alcohols, and epoxy resins with a cross-linking agent withfluxing properties.
 13. The solder composition of claim 1 wherein thesecond fluxing agent includes at least one of an organic acid with twoor more carboxylic groups and an epoxy resin with a cross-linking agentwith fluxing properties.
 14. The solder composition of claim 1 whereinthe second fluxing agent comprises an agent selected from a groupconsisting of aromatic dicarboxylic acids having 6–30 carbon atoms andaliphatic dicarboxylic acids having 6–30 carbon atoms.
 15. A soldercomposition adapted to couple a microelectronic component to asubstrate, comprising: a metal alloy; a binder; a rheology modifier; afirst fluxing means for reacting with an oxide of the alloy to promote afirst melting of the alloy at a first temperature; and a second fluxingmeans for reacting with an oxide of the alloy to promote a secondmelting of the alloy at a higher second temperature, which secondmelting occurs after the first melting of the alloy and a subsequentresolidification of the alloy.
 16. The solder composition of claim 15wherein the binder comprises a rosin or a resin adapted to flow at atemperature less than the first temperature.
 17. The solder compositionof claim 15 wherein the second temperature is at least about 25° C.higher than the first temperature.
 18. The solder composition of claim15 wherein the second temperature is about 25–70° C. higher than thefirst temperature.
 19. The solder composition of claim 15 wherein thefirst temperature is no higher than about 240° C.
 20. The soldercomposition of claim 15 wherein the first temperature is about 150–220°C.
 21. The solder composition of claim 15 wherein the second temperatureis at least about 200° C.
 22. The solder composition of claim 15 whereinthe second temperature is about 220–270° C.
 23. The solder compositionof claim 15 wherein the first fluxing means comprises an agent selectedfrom a group consisting of organic acids, amines, alcohols, and epoxyresins with a cross-linking agent with fluxing properties.
 24. Thesolder composition of claim 15 wherein the second fluxing means includesat least one of an organic acid with two or more carboxylic groups andan epoxy resin with a cross-linking agent with fluxing properties. 25.The solder composition of claim 15 wherein the second fluxing meanscomprises an agent selected from a group consisting of aromaticdicarboxylic acids having 6–30 carbon atoms and aliphatic dicarboxylicacids having 6–30 carbon atoms.